The present invention concerns a new method to determine if and when a solvent which, together with the substance being analyzed, constitutes the sample injected into a gas-chromatographic analysis apparatus, undergoes a change of state (evaporation or recondensation) inside a capillary tube and/or in correspondence to one or more pre-set points of the same. The invention also concerns methods to control and optimize the injection and introduction conditions of a sample into the analytical column of a gas-chromatographic apparatus, on the basis of information obtained by means of one or more detections indicating whether or not the above-mentioned change of state has occurred, as well also a device and an apparatus to obtain said detections and perform said control.
As it is known, a gas-chromatographic analysis apparatus generally consists of an injector, an analysis column, an oven or chamber housing the column, and a detector. The most recent technical advances in this field have focused mainly on the phase preceding the true analysis, that is on the phase of injection of the sample (defined as the substance to be analyzed, diluted in a suitable solvent) which, pushed by a carrier gas, must be transferred to the analysis column under optimal conditions.
To this end, a variety of techniques have been developed, mainly and more generally including vaporization injection techniques and so-called direct on-column injection techniques.
According to the former techniques, the sample, introduced in a liquid state, vaporizes in a special, independently-heated chamber, and is then fed, in whole or in part, to the column or better to a so-called pre-column (with or without a stationary phase), inside of which pre-determined conditions are createdxe2x80x94in particular in order to obtain that the sample undergoes or not (depending on the chosen operational methods) to a recondensation. The pre-column is usually housed in the same gas-chromatographic oven which houses the column, but a pre-column dedicated oven could be foreseen. Furthermore, when the sample undergoes recondensation, it is suitable to eliminate most of the vaporized solvent before the sample enters the analysis column, which can be performed by means of a discharge duct controlled by a valve and positioned downstream the pre-column and upstream the introduction point in the analysis column. This is especially important when, according to the most recent techniques, injections are performed with large volume samples.
In any case, these techniques require a detection that sample recondensation be effectively carried out, or not, in the column or pre-column, by determining in all cases the sample conditions, specially sample temperature conditions inside the column or pre-column, in order to obtain or not said sample recondensation.
Furthermore, it is often necessary to ensure that the pre-column temperature is very close to that causing the recondensation, i.e. slightly above or slightly below said temperature, in order to ensure the best analytical results.
When most of the solvent vapors are discharged downstream the pre-column, it is crucial that the closing time of the discharge valve is precisely choosen in order to avoid, on one hand, excess solvent in the column, resulting in longer and unuseful analysis times and, on the other hand, losing the most volatile compounds.
Finally, with some injection techniques, it is important to ensure that the length of column or pre-column which is wetted by the sample in its liquid state has a pre-determined size: in other words, it is important to know the quantity of liquid present in the column or pre-column, which obviously depends from the length of wetted capillary tube.
In order to obtain the best conditions for injecting and/or introducing the sample, it is of the utmost importance to be able to establish if and under what conditions the change of state (i.e. liquid-to-vapor or vapor-to-liquid) of the solvent or sample effectively takes place, either in the column or pre-column, or even in correspondence to one or more points along same, so as to use the information thereby obtained to optimize the control conditions of injection and/or introduction of the sample into the column.
Therefore, a main object of the invention is to provide a method for detecting the change of state of a solvent or sample (substance to be analyzed+solvent) inside a capillary tube or in one or more points along the same, said capillary tube being in particular a pre-column or a column belonging to a gas-chromatographic apparatus.
Another object of the invention is to provide a method for controlling the injection conditions of a sample in a gas-chromatography, so to optimize these conditions on the basis of information obtained by the afore-mentioned change of state detection.
Yet another object of the invention is to provide a device to detect said change of state inside a capillary tube or in one or more points along same, as well as a gas-chromatographic analysis apparatus, including at least one of the afore-mentioned detection devices and means to utilize the related information in order to optimize sample injection and/or introduction conditions in the same apparatus or in similar ones.
Finally, another main object of the invention is to provide, by means of the above-cited method and device, a behaviour model for the sample in a gas-chromatographic pre-column or column, according to the solvent used. This model can be expressed in the form of equations whose constants and/or parameters are experimentally determined and can be put into a software form, so that it may be used for analysis conducted on apparatuses without said means to detect the above-mentioned change of state.
According to the invention, a method is provided to detect any change of state (recondensation or evaporation) that occurs in an injected solventxe2x80x94in case together with a substance to be analyzedxe2x80x94within a capillary tube of a gas-chromatographic apparatus, said method being characterized by the steps of measuring the temperature in correspondence of at least one point of said capillary tube, and/or the pressure, or flowrate of the carrier gas fed to said capillary tube, and of registering changes in said temperature, and/or in the gas pressure, or flowrate, which are detected when a solvent change of state at every temperature change detecting point and/or within the capillary tube occurs.
In particular, measurement of changes in pressure or flowrate of the carrier gas is carried out upstream of the capillary tube and indicates a total evaporation of the liquid in the capillary tube. A measure of temperature changes, on the other hand, is carried out by means of a thermal probe applied to the outside of the capillary tube, in correspondence to every point to be detected.
Furthermore, always according to the invention, in order to control and optimize the injection and/or introduction conditions of a sample, as formed by a substance to be analyzed dissolved in a solventxe2x80x94in a gas-chromatographic analysis apparatus comprising an injector, an eventual pre-column, a separation column, a detector, an oven or chamber for the separation column, and an oven or chamber for the pre-column, coincident with or separate from the analysis column ovenxe2x80x94at one or more pre-fixed points on the column or pre-column or respectively upstream of the same the external temperature of the column or pre-column, and/or the pressure, or flowrate of the carrier gas is measured and at least one of the parameters controlling the injection and/or introduction into the separation column is modified following the detection of changes in temperature, and/or pressure, or flowrate and consequently a change of state of the solvent or sample at said point or points of the column or pre-column, and/or in the column or pre-column itself, according to the above described method.
Always according to the invention and in order to control the sample introduction conditions in a gas-chromatographic analysis apparatus, as above defined, it is possible to measure, at one or more pre-fixed points on the column or pre-column, the external temperature of same, to vary at least one of the injection parameters until a change in temperature and then a change of state of the solvent or sample is detected at said point, according to the method described above, and modify at least one of the apparatus functional parameters on the basis of the information obtained.
Furthermore, according to the invention a method is provided for controlling and optimizing injection and/or introduction conditions of samples into the analysis column of a gas-chromatographic analysis apparatus, characterized by the creation of a mathematical model of the solvent or sample behaviour in a similar apparatus, by means of experimental detection of temperature changes in one or more points on the column or pre-column, and/or of pressure, or flowrate changes within the same, according to the described method, for each solvent in a range of solvents; and by the transformation of said mathematical model in a software that can be applied to the original gas-chromatographic analysis apparatus, without further measure of temperature, and/or pressure, or flowrate changes.